Poly(1,4-butylene terephthalate) is a widely used molding resin because of its rapid crystallization and also because of its rigidity, good dimensional stability, low water absorption and good electrical properties. The resin also has a high heat resistance, inherent lubricity and excellent chemical resistance. One restriction on the use of this valuable resin, however, is the fact that the impact strengths of moldings tend to be somewhat inadequate for applications where the molded part is likely to be subjected to severe service conditions. Another important restriction is a lack of elastic properties, which limits the utility of poly-(1,4-butylene terephthalate) in making good fibers and films. Both disadvantages are overcome by providing block copolyesters in which one segment is poly(1,4-butylene terephthalate) and the other comprises polytetramethylene oxide (U.S. Pat. Nos. 3,651,014 and 3,766,146) or poly(1,4-butylene terephthalate-co-o-phthalate) (U.S. Pat. No. 4,096,126), or poly(hexylene azelate-co-isophthalate), etc. (U.S. Ser. No. 752,325, filed Dec. 20, 1976). Each of the patents and the application is incorporated herein by reference. The copolyester having the aliphatic ether segments has been reported to have thermal stability and weatherability shortcomings. The random copolyester having orthophthalate segments is slow to crystallize and difficult to made and use because orthophthalic anhydride sublimes from it at elevated temperature. The copolyester with aromatic-aliphatic or aliphatic acid segments has relatively low elastic recovery in comparison to the other polymers mentioned.
It has now been discovered that if a poly(1,4-butylene terephthalate) resin is chemically modified by being segmented in a copolyester in which a first portion of the repeating units are poly(1,4-butylene terephthalate) blocks and a second portion of the repeating units are blocks of a polyester of a cycloaliphatic 1,2-dicarboxylic acid or a derivative thereof and 1,4-butanediol, then the resulting copolyesters will have enhanced impact resistance, compared to the resin itself, and improved crystallization rate and stability, compared to other copolyesters mentioned. The improvement in impact resistance is achieved with minimal loss of other physical properties and is accompanied with a measurable increase in toughness. It has also been discovered that the presence of the internal units of the other polyesters modifies the rate at which poly(1,4-butylene terephthalate) crystallizes from the melt in a very desirable manner.
If the polyesters are added to the reactor during the preparation of poly(1,4-butylene terephthalate) after ester interchange between dimethyl terephthalate and 1,4-butanediol, there is caused a most desirable modification in the properties of the resulting polyester resins.
After completion of the reaction and molding the copolyesters, the moldings are improved in toughness and reduced in notch sensitivity as compared to bars molded from unmodified poly(1,4-butylene terephthalate). Even at only 10% of the second polyester content, the increase in impact strength is so marked that some of the samples cannot even be broken.
In addition to their use in injection molding applications, the polyester coreactants have also been found to be beneficial in improving the properties of poly(1,4-butylene therephthalate) resins used in other applications, such as profile extrusion, extrusion- and injection blow molding, thermo-forming, foam molding; in these cases small amounts of ester-forming branching agents may be added to enhance the melt elasticity properties of the products for easier processing.
The copolyester products have also been converted to valuable modifications by adding reinforcements and/or fillers, such as glass fibers, talc, clay, mica, and the like. Surprisingly, the increased toughness of the new copolyesters compensates for the greater brittleness usually induced by the incorporation of such non-soluble additives and fillers.